V3
-NOT MULTIPLAYER SAFE - BACK UP YOUR UPDATE.RPF FILE AND RESTORE IT BEFORE ATTEMPTING TO PLAY MULTIPLAYER-
Note: Crashes have been reported. GTAV modding is currently in it's early stages and as far as I know there's nothing I can do about it at the moment. It's still good for novelty value or a couple rampages, but I wouldn't use it during a mission. It's possible that the game engine can't handle there being more cars than normal. More pedestrians works fine.

Possible crash fix: After dragging the file into openIV, go to file menu and select "rebuild" then rebuild the file. This should hopefully improve stability. Special thanks to amishhipster for this potential fix.

This mod adds 2x the traffic and up to 4x the pedestrians depending on which file you use. The city feels so much more alive with this mod on!
Comes with several versions, 2x traffic 2x pedestrians, 2x traffic and 3x pedestrians, stock cars 4x peds, 4x cars 4x peds, no cars (only parked) no peds. Most of the screenshots are from the 2x traffic 3x peds version.
I didn't notice any performance hit, but less powerful computers may have issues with this mod.

Drive safely!
Feel free to upload this to other sites or modify it. Credit would be nice but not required.

Installation:
Backup your update.rpf file (especially if you ever want to play MP again)
Open update.rpf using the latest OpenIV
Navigate to common > data in the file
Enable edit mode, then drag and drop gameconfig.xml over the OpenIV window.
Go to OpenIVs file menu, select rebuild then rebuild the rpf.

DO NOT USE IN MP

Updates:
V1:
First version
V2:
Slight changes to the code
Added "Gridlock" config. 4x cars and pedestrians. Great for screenshots.
Added "Omega man" config. No cars or pedestrians. Once in a while you'll find a single driving car, can't seem to stop that.
Added Normal traffic 4x peds config. I've heard that it's more stable like this.
Added backup config file.
Added "New York" 8x peds/traffic config. Very unstable, but good for screenshots.
Added backup config file.
V3:
Added 2x cars 4x peds.

Future:
Stability fixes once the modding scene gets its bearing.
settings.xml?